In recent years, what is called the knocking feedback system for detecting a knocking which occurs in an internal combustion engine and for retarding the ignition timing in response to the detection has been studied from various angles. The knocking feedback system will be described briefly below.
The variations of noises caused in or outside of a cylinder due to the pressure in the cylinder of the internal combustion engine are detected, and when these vibrations or noises, as the case may be, exceed a set level (a knocking decision level), it is decided that a knocking has occurred so that a knocking signal is generated. Upon generation of the knocking signal, the ignition timing is retarded, while in the absence of the knocking signal, the ignition timing is advanced. In this way, the ignition timing is always controlled at a value near the knocking limit, thus improving the fuel economy and the output performance of the engine.
In this knocking feedback system, the knocking limit to which the ignition timing is controlled is subject to a change considerably toward the retard side according to the environmental conditions. As a result, the ignition timing is also controlled on the retard side. If the ignition timing is controlled to the retard side excessively, however, the temperature rise of the exhaust gas of the engine may damage the engine. Therefore, the lower limit of the ignition timing is required to be restricted. The conventional knock feedback systems are such that in the case where the knocking limit is moved to the retard side and the ignition timing is controlled to the retard side, the ignition timing is fixed to the lower limit. Nevertheless, the retard compensation representing the retard from the basic ignition timing determined in accordance with the engine speed and load is added by a predetermined retard amount up to a set maximum retard amount. This is because of the fact that even when the ignition timing has reached a lower limit, the retard compensation amount has not always reached the maximum retard amount. In the event that the knocking limit changes from this state toward the advance side with the change of the engine conditions or the like, the ignition timing fails to follow the change immediately and lags behind the change by the excessive retard compensation amount. On account of this time lag, the fuel efficiency and the output performance of the engine are adversely affected.
An object of the present invention is to provide a method and an apparatus for ignition timing control of the internal combustion engine, wherein in view of the above-described problem points, the retard compensation amount is not added upon detection of a knocking but the ignition timing is caused to follow the change of the knocking limit in the case where the knocking limit is located to a considerably retard side and the ignition timing has reached the lower limit for protection of the engine.
The present invention will be described with reference to FIG. 1. In FIG. 1(a), assume that the knocking limit A has changed to B on retarded side. The retard compensation amount increases as shown in FIG. 1 (c), and the ignition timing is retarded as shown in FIG. 1(b) (The dashed line indicates the case of the prior art system). When the ignition timing has reached the lower limit H, the ignition timing is fixed thereto. Under this condition, the retard compensation amount increases only up to J but not beyond. When the knocking limit changes from B to C toward the advance side, the ignition timing is rapidly advanced to follow the new knocking limit on account of the fact that the retard compensation is located at point J.
According to the present invention, the retard compensation amount representing the retard from the basic ignition timing is not added any more, in the case where the knocking limit is located considerably to retard side and the ignition timing is fixed at the lower limit. In other words, in the case where the ignition timing is fixed to the lower limit as mentioned above, the retard compensation amount is set to the difference between the basic ignition timing and the lower limit of the ignition timing (FIG. 2). Accordingly, the present invention prevents the failure of the ignition timing to follow the change of the knocking limit which otherwise might occur on account of an extraneous retard compensation amount, with the result that the ignition timing is caused all the time to follow the change of the knocking limit, thereby preventing the fuel efficiency and output performance from being deteriorated.